How to remove unwanted textures to save memory
Do not worry about all the images in your blend file. If they are not used in the render they will not effect the memory. Simply change the images out for the smaller ones and try to render again (make sure your nodes are not using the large image anywhere).

To remove the images from the blend file Shift LMB on the X in the image datablock. (Image from the UV Editor header)

How do I completely remove an image from my .blend file?

If you still are out of memory, then you are going to have to look at you geometry. Do you have any hidden geometry? Subdivision surface jacking up the poly count? Many of the same objects not sharing a datablock.